1. Field of the Invention
The invention relates to a process for the selective hydrogenation of hydroformylation mixtures which are produced in the preparation of higher oxo alcohols by hydroformylation of the corresponding olefins. The hydrogenation is selective to the extent that the aldehydes and certain byproducts of the hydroformylation are hydrogenated to give the desired alcohols, whereas the unreacted starting olefins are retained virtually completely.
2. Discussion of the Background
Higher alcohols, in particular those having from 6 to 25 carbon atoms, are known to be able to be prepared by catalytic hydroformylation (or oxo reaction) of the olefins having one carbon atom less and subsequent catalytic hydrogenation of the aldehyde- and alcohol-containing reaction mixtures. They are predominantly used as starting materials for the preparation of plasticizers or detergents.
It is known that in the catalytic hydroformylation of olefins, reaction mixtures are formed which, apart from the desired products, i.e. aldehydes and the corresponding alcohols, depending on the catalyst and the reaction conditions, can contain, in addition to unreacted olefins, by-products and secondary products of the hydroformylation, such as saturated hydrocarbons resulting from the olefins by hydrogenation, water, esters of the desired alcohols (e.g. formates), acetals of the target products aldehyde and alcohol, enol ethers and other by-products or secondary products. Said substances can be subdivided into low-boilers having a boiling point below the boiling point of the aldehyde and high-boilers having a boiling point above the boiling point of the alcohol. During the hydrogenation of the reaction mixtures, the desired alcohols are formed from some of the byproducts, such as esters and acetals, which improves the yield. On the other hand, it is desirable that the unreacted olefins remain in the hydrogenation so that they can be recirculated to the hydroformylation reaction, after separating them off from the hydrogenation mixture. Obviously, the olefins could already be separated off prior to the hydrogenation of the hydroformylation mixture by distillation of the aldehydes and the alcohols. However, this would mean an additional process step, since the hydroformylation mixture must be distilled in any case after the hydrogenation of the aldehydes.
The catalytic hydrogenation of reaction mixtures which were prepared by cobalt-catalyzed hydroformylation of olefins having from 2 to 24 carbon atoms is described in DE 35 42 595. The hydrogenation is carried out in two stages. In the first stage, the hydroformylation mixture is hydrogenated at 150-230.degree. C. and a hydrogen pressure of 10-350 bar with 80-95% conversion on a supported SiO.sub.2 catalyst which comprises 5-15% by weight of nickel and 2-20% by weight of molybdenum in the form of molybdenum oxide. In the second stage, the hydrogenation is completed at 150-230.degree. C. and 10-350 bar hydrogen pressure on a catalyst whose active mass consists of 55-60% by weight of cobalt, 15-20% by weight of copper, 4-10% by weight of manganese and 2-5% by weight of molybdenum in the form of molybdenum oxide and, if appropriate, up to 10% by weight of activating additives. In the process, the formates and acetals present in the mixture are converted to the corresponding alcohols. The process is obviously not aimed at a selective hydrogenation with retention of the olefins, since these are not mentioned at all. Furthermore, a disadvantage in the process is that the hydrogenation is carried out in two stages and at high pressures, according to the example at 250 or 245 bar.
According to U.S. Pat. No. 5,399,793, for the hydrogenation of decobalted reaction mixtures, as arise in the hydroformylation of C.sub.5 -C.sub.12 olefins, use is made of Ni/Mo catalysts on Al.sub.2 O.sub.3 or A.sub.2 lO.sub.3 SiO.sub.2 as support materials. The total process comprises the following individual steps:
(a) cobalt-catalyzed hydroformylation PA1 (b) decobalting of the reaction mixture PA1 (c) hydrogenation of the crude reaction mixture at elevated temperature and at elevated pressure PA1 (d) production of alcohols having very low amounts of aldehydes by distillation and PA1 (e) finish-hydrogenation of the alcohols.
The hydrogenation of stages (c) and/or (e) can be carried out using a bimetallic, phosphorus-free Ni/Mo hydrogenation catalyst. This hydrogenation catalyst produces fewer high-boiling byproducts than a corresponding phosphorus-containing catalyst. Although, in the examples, the presence of low-boilers, that is to say olefins and paraffins, is mentioned, no information is given on the mass ratio of these substances before and after the hydrogenation. It is a disadvantage in any case that to prepare an on-specification alcohol which is suitable for preparing plasticizers, two hydrogenation stages are necessary and that at least in the hydrogenation stage (b) a relatively high pressure of 1000 psig (about 70 bar) is necessary.
In addition, processes have become known in which a compound which contains a carbonyl function and an olefinic double bond is hydrogenated catalytically to the corresponding alcohol selectively retaining the olefinic double bond. Thus, according to Japanese patent application SHO 57-110354, 7-octenal is selectively hydrogenated to 7-octen-1-ol at from 70 to 150.degree. C., use being made of a chromium oxide catalyst or a catalyst which consists of at least two of the metals chromium, copper and tin. However, this process has the disadvantage that a solvent is used which must be separated off again. In addition, the achievable space-time yields in the temperature range of from 70 to 130.degree. C. are too low for an industrial application. At higher temperatures at which the hydrogenation rate is higher, the hydrogenation selectivity decreases rapidly owing to the unwanted hydrogenation to the saturated alcohol. At 140.degree. C., the hydrogenation selectivity to the unsaturated alcohol is already below 95%.
In addition, citronellal may be hydrogenated to citronellol, retaining the olefinic double bond. For this purpose, according to Applied Catalysis, 25 (1986),181-189, use is made of ruthenium catalysts and a yield of up to 90% is achieved. Using Cu/Cr catalysts, according to Iv.Akad.Nauk.Gruz SSR. Ser. Khim.16(4) (1990), 287-292, a yield of 92% was achieved.
On the other hand, it is known that 2-ethylhex-2-enal, when use is made of a Cu/Cr/Ni catalyst which contains an alkali metal component, can be hydrogenated to 2-ethylhexanol (EP O326 674 A2). In this case, not only the carbonyl function but also the olefinic double bond are hydrogenated.
The object underlying the present invention was to hydrogenate reaction mixtures of the hydroformylation of C.sub.5 to C.sub.24 olefins under comparatively mild conditions and, in particular low pressures, selectively in such a manner that the aldehydes and certain accompanying substances present in addition to alcohols and aldehydes, in particular formates, are converted, as substantially as possible, into the desired alcohols and the unreacted olefins are hydrogenated as little as possible.